1. Field of the Invention
The present invention relates to optical devices. More specifically, the present invention relates to Faraday rotators.
2. Description of the Related Art
Faraday rotators are used in laser systems to provide an optical isolator that allows light to travel in one direction and block light in the other direction. Optical isolators are useful in high power lasers to allow a low power laser to feed a high power amplifier and block the resulting high power beam from feeding back into the low power laser beam. This is particularly important in weapons class lasers inasmuch as these devices may operate at hundreds of kilowatt levels.
Faraday Rotators are a type of optical device that provides a non-reciprocal rotation of the polarization of a light beam passing through the Faraday Rotator. The non-reciprocity of the polarization rotation allows creation of an optical isolator by placing a Faraday rotator with a 45 degree rotation between a pair of polarizers whose axes are rotated by 45 degrees with respect to each other. Faraday isolators act as optical diodes allowing a signal to pass with low loss in one direction while creating a high attenuation for signals traveling in the reverse direction. Faraday rotators consist of an optical material with a high Verdet constant that is placed in a strong magnetic field. The Faraday effect causes light traveling along the direction of the magnetic field to experience polarization rotation.
Historically, use of Faraday rotators at high power levels (greater than a few 10s of watts) has been problematic because the optical materials used in Faraday rotators generally have some level of intrinsic residual absorption. At high average power levels, the heating due to this absorption in materials with useful Verdet constants causes a number of detrimental effects including thermal lensing: thermal gradients that lead to undesirable optical lensing effects, thermally induced stress birefringence, and thermally induced drift in the amount of rotation provided by the Faraday Rotator. These thermal effects degrade the effectiveness of the optical isolator, degrade the beam quality of the laser beam passing through the isolator, and/or result in optical damage to the isolator. Hence, there is a need for a means for managing the heat loads in Faraday rotators at high power levels in a way that minimizes these deleterious effects.
Prior efforts to develop Faraday rotators capable of high power operation have focused on minimizing intrinsic absorption in the optical medium, configuring the optical medium as a rectangular slab in order to improve cooling and reduce thermal gradients, and compensating for birefringence by using two 22.5 degree Faraday rotators with a 90 degree reciprocal rotator between the Faraday rotators similar to Scott-DeWitt compensation applied to correct for birefringence in laser rods. The combination of all of these approaches by Khazanov has yielded Faraday Isolators with power handling up to 2 kW at greater than 20 db extinction. Unfortunately, this approach leads to rather large isolators and is unlikely to scale to 10s of kW.
Hence, a need remains in the art for a Faraday rotator capable of operating at much higher power levels without performance degradation due to thermal gradients.